Rozwój sedymentacji w dolnej jurze Wyżyny Krakowsko-Wieluńskiej i pozycja stratygraficzna osadów gruboklastycznych

Abstract

Lower Jurassic sedimentation history of Cracow-Wieluń Upland and stratigraphic position of coarse clastic deposits Sandy-gravel deposits attributed to Lias are widespread over the area of the Cracow—Wieluń Upland (Fig. 1). They have been considered up to now by most authors as one formation of the Middle Lias age and informally named Połomia Beds (Znosko, 1955). During the sedimentological investigations of these deposits carried out by the present author they were found to consist of three different sedimentary series, distinguished mainly due to differences in the rock composition of the gravels. Their succesion was established on the grounds of changes in the rock composition. Their age was determined by a comparison with the fully developed liassic deposits in the northern part of Poland. In the first series of the sandy-gravel deposits the petrographical composition of the gravels is the simplest (Fig. 2). It is parallelised with the Gorzów Beds described by Znosko (1955) and by an analogy considered to be of a Rhaetian age. These deposits were eroded during the Lias and their material was included into the liassic deposits. The second series is somewhat more differentiated in the petrographical composition (Fig. 3) but still rather poor. Due to the striking similarity of these deposits to the earlier described (Rutkowski, 1923; Znosko, 1953; Mossoczy, 1961) so called Sub-coal Beds the described series could be attributed to this informal lithostratigraphic unit. Their deposition has taken place in the Middle Lias time according to most authors (Znosko, 1959; Mossoczy, 1961; Dadlez, 1964, 1969; Jurkiewiczowa, 1967) as a result of the Domerian sea regression. The third series of the coarse clastics has a very rich petrographical assemblage (Fig. 4) strongly contrasting with the two older series. It is formally defined as the Połomia Formation; the geographical component of the formation name being the same as that of the traditional informal division. As it was ascertained that these deposits are younger then the sub-coal Beds (Middle Lias), their deposition is supposed to have taken place in the late Toarcian time, when there existed optimum conditions for the coarse clastic deposition due to a marine regression. The Połomia Formation is a facies equivalent of the upper Toarcian Łysieć Beds developed in the north-east vicinity as well as of the Kamień Beds (Dadlez, 1969) of the western part of Poland. The Połomia Formation is a deposit of a braided river. Such conditions were related to the climatic changes caused by a sea regression and an uplift of the source area. The palaeoslope was determined for different sections of the river course by a comparison of the grain-size and sedimentary structures of the deposits of the given sections with the sediments of the actual rivers. The palaeoslope estimates led to the conclusion that the river valley bottom in the source area was elevated at least 800 m above the contemporaneous sea level in the Late Toarcian time. The source areas (Fig. 5) were identified on the grounds of the rock composition of the gravels combined with the palaeotransport measurements.